1. Field of the Invention
The present invention relates to a novel dielectric material composition which comprises a metal oxide having a high dielectric constant and the properties of ferroelectricity, piezoelectricity, and pyroelectricity. The dielectric material composition can be applied in a wide range of various fields such as memory devices, microwave devices, sensors, microelectromechanical devices, and optoelectric devices.
2. Description of the Related Art
Some single crystal or poly-crystal metal oxides, such as BaTiO3, LiNbO3, Pb(Zr,Ti)O3, and (Sr,Ba)Nb2O6, have ferroelectric properties and are widely applied in electronic, optoelectric, non-linear optical, and piezoelectric devices, such as optical waveguides, optical modulators, ferroelectric thin film optical memory and displays, electroacoustic transducers, high frequency surface(bulk) acoustic wave devices, infrared sensors, ferroelectric nonvolatile memory devices.
There are three types of dielectric materials having ferroelectric properties. The first type has a pervoskite structure with a general formula ABO3, wherein BaTiO3 is a representative type. The second type is tungsten bronze material, such as SBN ((Ba,Sr)Nb2O6) and PBN((Ba,Pb)Nb2O6). The third type is YMnO3 type material with a general formula RMnO3 having a hexagonal or rhombic crystalline structure, wherein R is a metal selected from rare earth elements, such as scandium (Sc) and Yttrium (Y). For example, YMnO3, HoMnO3, ErMnO3, YbMnO3, TmMnO3, LuMnO3 and solid solutions thereof are hexagonal crystals having ferroelectric properties. The current ferroelectric materials of interest, PZT and SBT, are classified as the first type, or pervoskite structure, and have advantageous and disadvantageous properties. PZT has a relatively low process temperature and a high remnant polarization (PZT: 600˜700° C./15˜30 μC/cm2 versus SBT: 750˜800° C./5˜10 μC/cm2), but SBT has a relatively low coercive field and a good fatigue resistance (PZT: 50˜70 kv/cm versus SBT: 30˜50 kv/cm). Accordingly, it is necessary to develop a ferroelectric material composition having a new structure and the same advantages as PZT and SBT, for the exploitation of applicable range and field. There are many investigations directed to the adjustment of the ABO3 pervoskite structure utilizing the theory of valence change and the principle of electrical neutrality. The examples are A1+ B5+ O3, A2+ B4+ O3, A3+ B3+ O3, AxBO3, A(B′0.67B″0.33)O3, A(B′0.33B″0.67)O3, A(B0.5+3 B0.5+5)O3, A(B0.52+B0.56+)O3, A(B0.51+B0.57+)O3, A3+(B0.52+B0.54+)O3, A(B0.251+ B0.755+)O3, A(B0.53+B0.54+)O2.75, and A(B0.52+B0.55+)O2.75. Nevertheless, the adjustments for the third type of ferroelectric materials mentioned above are mostly directed to R in RMnO3, rarely to Mn.
U.S. Pat. No. 5,919,515 discloses a method for preparing a ferroelectric film formed on a substrate, the ferroelectric film having a YMnO3 hexagonal crystal structure of, and composed mainly of manganese, oxygen and at least one element selected from the group consisting of rare earth elements, scandium and yttrium.
U.S. Pat. No. 5,985,404 discloses a ferroelectric recording medium for recording information by utilizing the polarization reversal of a ferroelectric material, comprising a ferroelectric layer of a pervoskite material, a tungsten bronze material, or an oxide material containing a rare earth element (inclusive of scandium and yttrium), manganese and oxygen and having a hexagonal YMnO3 crystalline structure.
U.S. Pat. No. 5,955,213 discloses a ferroelectric thin film formed on a substrate, wherein the film has an AMnO3 hexagonal crystal structure and comprises manganese, oxygen, and element A, wherein element A is selected from the group consisting of a rare earth element, scandium, and yttrium.
U.S. Pat. No. 5,138,520 describes a general composition of ABO3 which includes PbTiO3, PbxZrxTiO3, PbxLayZrzTiO3, and YMnO3 where Y represents any rare-earth element, and TiYMnO3.
The metal oxides in the patents mentioned above are different from Y1−xM1xMn1−yM2yOm of the present invention.